[Epic] Raft: Pre-Vote mechanism

[veod32]

Dev board ref.: https://github.com/tarantool/roadmap-internal/issues/92
PM ref.: https://www.notion.so/tarantool/RAFT-95ae32cc025f465e8f3d25177efcd291

SME: @ sergepetrenko

Tasks

• [1sp] Raft: document the new Pre-Vote stage in leader election process #2593
• [2sp] elections: new box.info field and binary protocol changes #2837

Details

To document the implementation of the new Pre-Vote mechanism in Tarantool automated leader election.

[patiencedaur]

Here is a source—a conversation on Raft between @ rosik and @ sergepetrenko: https://hackmd.io/@rosik/2021-08-25-raft-interview